The present invention is directed to a method of forming catalytic materials in such a manner that the catalytic material does not substantially undergo capillary condensation. Accordingly, the adverse effect that water vapor has on catalytic activity of the catalytic material is minimized.
Catalytic materials, especially for removing pollutants from a pollutant-containing gas are generally comprised of metals as well as other constituents which actively induce a chemical reaction. The effectiveness of a catalytic material depends in part on the availability of catalytically active sites. The more catalytically active sites available from a given catalyst, the more efficiently the catalytic material can promote the desired reaction.
Catalytic materials are used to induce the reaction of pollutants contained within a pollutant-containing gas into harmless by-products. There are numerous pollutants which are found in the atmosphere and/or contained within gas discharged from industrial facilities or automotive vehicles. Such pollutants include hydrocarbons, carbon monoxide, ozone, sulfur compounds and NO.sub.x compounds.
If a potentially catalytically active site is blocked then its availability to catalyze the chemical reaction of a pollutant is eliminated or at least substantially reduced. Compounds which block catalytically active sites do so by binding to the catalytic material so that at least a portion of the time the catalytically active site is unavailable for catalyzing a reaction. The stronger the bond between the blocking compound and the catalytically active site, the less efficient the catalytic material is in inducing a chemical reaction to convert pollutants to harmless by-products.
It is well known that water molecules have an affinity for catalytic materials, especially metals. Accordingly, water serves as a blocking compound which reversibly binds to catalytically active sites. The bond between water molecules and catalytically active sites is typically of moderate strength so that the water molecules spend only a portion of the time bound to the catalytically active site. When the water molecule is so bound, the particular catalytically active site is incapable of inducing a chemical reaction to convert pollutants to harmless by-products.
Catalytic materials including those incorporating precious metals, base metals and the like are employed in catalytic compositions for the treatment of pollutant-containing gases such as exhaust gas from automotive vehicles. The exhaust gases typically contain moisture or water vapor and the amount of water vapor will vary depending on climatic conditions. As previously indicated, the presence of water molecules can impede the effectiveness of a catalytic material because water acts as a blocking compound.
During normal operation of an automotive vehicle, the temperature of the exhaust gas will be several hundred degrees. Under these high temperature conditions, water molecules are energized due to the input of thermal energy. Highly energized molecules tend to remain in motion. This high energy level limits the time the water molecules remain bound to catalytically active sites. Accordingly, the presence of water vapor under high temperature operating conditions does not adversely affect the efficiency of catalytic materials to the same extent as under lower temperature operating conditions when water molecules are less energized. Under less energized conditions, water molecules tend to bind to catalytically active sites for a greater length of time than under high energy conditions (e.g. higher temperatures).
Catalytic materials are generally manufactured with a preference for high surface areas so as to enable a greater number of catalytic sites to catalyze the reaction of pollutants contained within a pollutant-containing gas. High surface area catalytic materials can be produced by employing a pore structure comprised of micropores having an average pore size as low as possible, typically less than 5 nanometers (nm). Smaller pores therefore, are characteristic of high surface area catalytic materials.
It has been observed that catalytic materials having an average pore size of less than about 5 nm, undesirably retain moisture especially under high humidity and low temperature (i.e. low energy) conditions. When water vapor is in contact with such materials, molecules of water enter the relatively small pores and remain within the pores. This phenomenon is known as capillary condensation.
"Capillary condensation" as used herein means that water molecules enter and remain within the micropore structure of the catalytic material. Because the micropores have very small pore sizes (typically less than 5 nm), the water molecules become "stuck" in the pores and can be removed only with some difficulty. The retention of water molecules in micropores (capillary condensation) reduces the effectiveness of catalytic materials because the water molecules block the catalytically active sites as previously described. In particular, the number of catalytically active sites available to catalyze the reaction of a pollutant is reduced and therefore the efficiency of the catalytic material is impaired.
It would therefore be a significant advance in the art of removing pollutants from a pollutant-containing gas to provide catalytic materials in which capillary condensation is prevented or at least substantially minimized. It would be another advance in the art to produce catalytic materials which can be used in automotive vehicles to remove pollutants from a pollutant-containing gas under high humidity and/or low temperature operating conditions.